Fluid-energy mills are used to reduce the particle size of a variety of materials such as pigments, agricultural chemicals, carbon black, ceramics, minerals and metals, pharmaceuticals, cosmetics, precious metals, propellants, resins, toner and titanium dioxide. The particle size reduction typically occurs as a result of particle-to-particle collisions and particle collision with the walls.
The fluid-energy mill typically comprises a hollow interior, the grinding chamber, where particle collisions resulting in grinding, occur. Within the grinding chamber, a vortex is formed via the introduction of a compressed gas or grinding fluid through fluid nozzles into the fluid-energy mill, wherein the fluid nozzles are positioned in an annular configuration around the periphery of the grinding chamber. The compressed grinding fluid (e.g., air, steam, nitrogen, etc.), when introduced into the grinding chamber, forms a high-speed vortex as it travels within the grinding chamber. The gas circles within the grinding chamber at a decreased radii until released from the grinding chamber through a gas outlet. The particles to be ground are deposited within the grinding chamber and swept up into the high-speed vortex, thereby resulting in high speed particle-to-particle collisions as well as collisions with the interior portion of the grinding chamber walls.
Particulate material and grinding fluid are introduced into the fluid-energy mill through a feed-inlet tube, which contains a feed nozzle for introduction of grinding fluid.
Typical nozzles that have been used include De Laval nozzles (converging-diverging nozzles) through which the grinding fluid (also known as compression gas) is injected into the grinding chamber. The particulate material is introduced into the feed-inlet tube from a chute. Particulate material distribution into the grinding fluid (for example, steam) can be irregular and results in unused grinding energy. In fact, the particulate material is found primarily concentrated along the feed-inlet tube wall such that the flow pattern is a core of grinding fluid surrounded by particulate material with limited mixing of the two. Particulate material introduced at low velocity from the feed chute is not likely to substantially penetrate the supersonic grinding fluid flow in this type of configuration. Consequently, the grinding occurs at the boundary between the particles and the high-velocity grinding fluid, also referred to as the shear zone. Thus, a sizeable portion of the kinetic energy of the grinding fluid is not utilized for grinding. As a result, a greater amount of energy is necessary and a greater volume of compression gas is required to grind the particulate material to the desired particle size. Energy efficiency would clearly improve if the available kinetic energy is more fully utilized through turbulent mixing of the particulate material and the grinding fluid.
In addition, turbulence is relatively less near the walls than in the core of the flow profile. Thus, larger agglomerates of particulate material are likely to pass through the feed-inlet tube into the grinding chamber of the fluid-energy mill without being ground to appropriate size.
Even a slight improvement in nozzle design would result in more effective use of energy in particle grinding and a significant reduction in steam consumption, thereby lowering variable production cost.
Thus, there is a need within the industry for a mechanism for reducing energy and grinding fluid consumption by increasing the mixing between the grinding fluid and the particulate material. The present invention addresses that problem in that particulate material is highly likely to get exposed to a high shear, high turbulence, region prior to entering the main body of the fluid-energy mill. For example, finished titanium dioxide pigment product for various uses, such as textiles, cosmetic additives, etc., requires a median particle size of ˜0.4 micrometer. Before grinding, the median particle size of the pigment particles and agglomerates is generally on the order of about 1 micrometer. Most of this grinding occurs in a relatively small section of the feed-inlet tube. It is well-acknowledged that particle comminution is both energy intensive and remarkably inefficient, with as little as 5% of input energy actually translated into particle size reduction. Given the tremendous inefficiencies in the particle size reduction process, a new supersonic feed jet nozzle design that more efficiently grinds titanium dioxide particles in the fluid-energy mill feed-inlet tube is desirable.
The present invention overcomes these problems in that it proposes placing an annular jet downstream of the primary jet in the feed-inlet tube for introducing a secondary grinding fluid that enables additional contact between the particulate material and the grinding fluids in a turbulent zone.